1. Field of the Invention
The present invention relates to a countergravity metal casting method and apparatus and, more particularly, to such a method and apparatus having displaceable mold portions for controlling the flow of metal into and out of the mold.
2. Description of the Related Prior Art
Vacuum-assisted, countergravity casting of molten metal has been successful in producing high-quality thin-walled castings. Examples of such processes are disclosed in the U.S. Pat No. 4,719,977, granted Dec. 20, 1988, To Chandley; U.S. Pat No.4,865,113, granted Sep. 12, 1989, to Voss et al.; and U.S. Pat No. 4,989,662, granted Feb. 5, 1991, to Spraw.
With these countergravity casting systems, a casting mold is formed with a feed gate extending up from the bottom of the mold and into a casting cavity therein. The feed gates are then placed in communication with an underlying metal source and a vacuum is applied to the casting cavity to draw the molten metal upwardly and fill the cavity. Since the metal is rising in the mold under low pressure from the bottom, excessive turbulence is avoided in the mold resulting in a cleaner, higher quality casting as compared to conventional gravity-cast molds.
A major drawback of known vacuum-assisted countergravity casting systems, however, is that the vacuum must be maintained until at least the metal in the feed gates solidifies or else the molten metal will drain out of the cavity upon discontinuing the vacuum. Consequently, the cycle time for producing a countergravity-filled casting typically exceeds the cycle time for producing a comparative gravity-fed casting.